Download Mobile Applications and Web Services

EEEM048- Internet of Things
Lecture 5- Software platforms and services
Dr Payam Barnaghi, Dr Chuan H Foh
Institute for Communication Systems (ICS)
Electronic Engineering Department
University of Surrey
Autumn Semester 2014/2015
1
Wireless Sensor (and Actuator)
Networks
Services?
End-user
Operating
Systems?
Core network
e.g. Internet
Gateway
Protocols?
Sink
node
Protocols?
Gateway
Computer services
- The networks typically run Low Power Devices
- Consist of one or more sensors, could be different type of sensors (or actuators)
Operating Systems
− An Operating System in an embedded system is a
thin software that resides between the node’s
hardware and the application layer.
− OS provides basic programming abstractions to
the application developer.
− The main task of the OS is to enable applications
to interact with hardware resources, to schedule
and pritorise tasks and to mediate between
applications and services that try to use the
memory resources.
3
Features of the OS in embedded
systems
−
−
−
−
−
Memory management
Power management
File management
Networking
Providing programming environment and tools
(commands, interpreters, compiler, etc.)
− Providing entry points to access sensitive
resources such as writing to input components.
− Providing and supporting functional aspects such
as scheduling, multi-threading, handling
interrupts, memory allocations.
4
Operating Systems and Run-time
environments
− Embedded operating systems
− Virtual machines
− Abstracting the hardware specific issues from the users.
− Need for energy-efficient execution
− The code is more restricted (compared to
conventional operating systems) so a full-blown
OS is not obviously required.
− An appropriate programming model
− A clear way to structure a protocol stack
− And support for energy management
5
Thread-based vs Event-based
Programming
− In WSN it is important to support concurrent tasks, in
particular tasks related to I/O systems.
− Thread-based programming uses multiple threads and a
single address space.
− This way if a thread is blocked by an I/O operation, the
thread can be suspended and other tasks can be executed
in different threads.
− The programmer must protect shared data structures with
locks, coordinate the execution of threads.
− The program written for multiple threading can be complex,
can include bugs and may lead to deadlocks.
6
Thread-based vs Event-based
Programming- II
− The event-based programming uses events and event
handlers.
− Event handlers are registered at the OS scheduler and are
notified when a named event occurs.
− The OS kernel usually implements a loop function that polls
for events and calls relevant event handlers when an event
is occurred.
− An event is processed by an event handler until completion
unless it reaches a blocking operation.
− In the case of reaching a blocking operation it registers a
new call back and returns control to scheduler.
7
Dynamic Programming
− There could be a requirement for programming some parts
of an application (for example in a WSN environment) after
deployment. Some of the reasons for this could be:
− The complete deployment setting and requirements may not
be know prior to the deployment and as a result the network
may not function optimally;
− Application requirements and physical environment properties
can change over time;
− There could be bug fixes or update requirements while the
network is till operating.
− In the above case manual replacement of software may not
be feasible because of the large number of nodes in the
network.
8
Dynamic Programming
− If there is no clear separation between OS and the
application dynamic programming can not be supported.
− In dynamic programming, OS should be able to receive the
software updates and stored in active memory.
− OS should make sure this is an updated version.
− OS should remove the piece of software that needs to be
updated from memory and should install and configure the
new version.
9
Embedded Operating Systems
− OS running on devices with restricted
functionality
− In the case of sensor nodes, there devices typically also
have limited processing capability
− e.g. TinyOS
− Restricted to narrow applications
− industrial controllers, robots, networking gear, gaming
consoles, metering, sensor nodes…
− Architecture and purpose of embedded OS
changes as the hardware capabilities change (i.e.
mobile phones)
Source: The Web of Things, Marko Grobelnik, Carolina Fortuna, Jožef Stefan Institute.
10
TinyOS
− “TinyOS is an open source, BSD-licensed
operating system designed for low-power
wireless devices, such as those used in sensor
networks .”
− TinyOS applications are developed using nesC
− nesC is a dialect of the C language that is
optimised for the memory limits of sensor
networks.
11
TinOS - programming
− “TinyOS is completely non-blocking:
− it has one stack.
− All I/O operations that last longer than a few hundred
microseconds are asynchronous and have a callback.
− To enable the native compiler to better optimize across
call boundaries, TinyOS uses nesC's features to link
these callbacks, called events, statically.”
TinyOS home page: http://www.tinyos.net/
TinyOS tutorial: http://docs.tinyos.net/tinywiki/index.php/TinyOS_Tutorials
12
TinyOS
− TinyOS does not provide a clear separation of concern
between the operating system and the application.
− Tasks, commands and events are fundamental building
blocks of the TinyOS run-time environment.
− Tasks are monolithic processes that should be executed
until they are complete.
− This means they can not be blocked by other tasks but they
can be interrupted by events.
− For example a packet reading task can schedule itself
repeatedly (sending an event signal) until it has read all the
packets.
− In TinyOS the scheduled tasks use a FIFO principle.
13
Contiki
− Contiki is the open source operating system for the Internet of
Things.
− runs on networked embedded systems and wireless sensor networks.
− It is designed for microcontrollers with small amounts of memory.
− A typical Contiki configuration is 2 kilobytes of RAM and 40
kilobytes of ROM.
− Contiki provides IP communication, both for IPv4 and IPv6.
− It has an IPv6 stack that, combined with power-efficient radio
mechanisms such as ContikiMAC, allow battery-operated devices to
participate in IPv6 networking.
− Contiki supports 6lowPAN header compression, IETF RPL IPv6 routing,
and the IETF CoAP application layer protocol.
− We will study 6LowPAN and CoAP protocols later in this module.
Source: http://www.contiki-os.org
14
Contiki- Functional aspects
− It’s kernel functions as an event-driven kernel; multithreading is
supported by an application library. In this sense it is a hybrid OS.
− Contiki realises the separation of concern of the basic system
support form the rest of the dynamically loadable and
programmable services (called processes).
− The services communicate with each other through the kernel by
posting events.
− The ContikiOS kernel does not provide any hardware abstraction;
but it allows device drivers and application directly communicate
with the hardware.
− Each Contiki service manages its own state in a private memory
space and the kernel keeps a pointer to the process state.
15
Contiki- Functional aspects
− Dynamic loading:
− Dynamic loading and reconfiguration of services is achieved by
defining services, service interfaces, service stubs and a
service layer.
− A Contiki service consists of a service interface and
implementation of it (which is also called a process).
− A service stub enables an application program to dynamically
communicate with a service through its service interface.
− A service layer is similar to a look up or registry for the
services.
− Programs call the services through their service interface and
stubs and there is no need for them to know about the
implementation details of the services.
− The loose coupling of a service with a program that uses the
service enables Contiki to update the service without the need
to modify the program of the application.
16
The Contiki OS
Loaded program
Loadable programs
Can be easily updated
Communication service
Core
Service
Language runtime
Program Loader
Core: single binary
Usually never modified
Kernel
17
Protothreads
− Protothreads can be seen as lightweight (stakless) threads.
− They can be also seen as interruptible tasks in event-based
programming.
− A protothread provides a conditional blocking “wait”
statement which takes a conditional statement and blocks
the protothread until the statement is evaluated true.
− By the time the protothread reaches the wait time if the
conditional statement is true, it continues executing without
any interruption.
− A protothread is invoked whenever a process receives a
message from another process or a timer event.
18
Protothreads- example
− For example consider a MAC protocol that turns off the
radio subsystem on a periodic basis; but you want to make
sure that the radio subsystem completes the
communication before it goes to sleep state.
1.
At t=t0 set the radio off
2.
The radio remains on for a period of tawake seconds
3.
Once tawake is over, the radio has to be switched off, but any on-going
communication needs to be completed.
4.
If there is an on-going communication, the MAC protocol will wait for
a period, twait_max before switching off the radio.
5.
If the communication is completed or the maximum wait time is over,
then the radio will go off and will remain in the off state for a period
of tsleep.
6.
The process is repeated.
Source: Adam Dunkels, Oliver Schmidt, Thiemo Voigt, and Muneeb Ali. 2006. Protothreads: simplifying event-driven programming of memoryconstrained embedded systems. In Proceedings of the 4th international conference on Embedded networked sensor systems (SenSys '06). ACM.
19
Radio sleep cycle code with events
Event driven code can be
messy and complex
Source: Adam Dunkels, Oliver Schmidt, Thiemo Voigt, and Muneeb Ali. 2006. Protothreads: simplifying event-driven programming of memoryconstrained embedded systems. In Proceedings of the 4th international conference on Embedded networked sensor systems (SenSys '06). ACM.
20
Radio sleep cycle with Protothreads
Source: Adam Dunkels, Oliver Schmidt, Thiemo Voigt, and Muneeb Ali. 2006. Protothreads: simplifying event-driven programming of memoryconstrained embedded systems. In Proceedings of the 4th international conference on Embedded networked sensor systems (SenSys '06). ACM.
21
Sensor Network Programming
− Sensor Network programming can be: node centric or it can
be application centric.
− Node-centric approaches focus on development of a
software for nodes (on a per-node level).
− Application-centric approaches focus on developing
software for a part or all of the network as one entity.
− The application centric programming will require
collaboration among different nodes in the network for
collection, dissemination, analysis and/or processing of the
generated and collected data.
− While in node centric programming the main focus is on
developing a software on a per-node level.
22
The Contiki code
Header files
#include "contiki.h"
PROCESS(sample_process, "My sample process");
Defines the name
of the process
AUTOSTART_PROCESSES(&sample_process);
PROCESS_THREAD(sample_process, ev, data) {
PROCESS_BEGIN();
while(1) {
PROCESS_WAIT_EVENT();
}
PROCESS_END();
}
Event parameter;
process can respond
to events
Threads must have
an end statement
Defines the
process will be
started every time
module is loaded
contains the
process code
process can receive
data during an event
System defined events in Contiki
−
−
−
−
−
−
−
−
−
−
−
PROCESS_EVENT_NONE
PROCESS_EVENT_INIT
PROCESS_EVENT_POLL
PROCESS_EVENT_EXIT
PROCESS_EVENT_SERVICE_REMOVED
PROCESS_EVENT_CONTINUE
PROCESS_EVENT_MSG
PROCESS_EVENT_EXITED
PROCESS_EVENT_TIMER
PROCESS_EVENT_COM
PROCESS_EVENT_MAX
24
The Contiki code
#include "contiki.h"
PROCESS(sample_process, "My sample process");
AUTOSTART_PROCESSES(&sample_process, &LED_process);
PROCESS_THREAD(sample_process, ev, data) {
static struct etimer t;
static int c = 0;
PROCESS_BEGIN();
etimer_set(&t, CLOCK_CONF_SECOND);
while(1) {
PROCESS_WAIT_EVENT();
if(ev == PROCESS_EVENT_TIMER) {
printf(“Timer event #%i\n", c);
c++;
etimer_reset(&t);
Process thread
names
Process thread 1
}
}
}
PROCESS_END();
PROCESS_THREAD(LED_process, ev, data) {
static uint8_t leds_state = 0;
PROCESS_BEGIN();
leds_off(0xFF);
leds_on(leds_state);
PROCESS_END();
}
Process thread 2
Running Contiki on a Hardware
− Write your code
− Compile Contiki and the application
− make TARGET=XM1000 sample_process
− Make file
CONTIKI = ../..
all: simple_process
include $(CONTIKI)/Makefile.include
− If you plan to compile your code on the chosen platfrom more
than once;
− make TARGE=XM1000 savetarget
− Upload your code
− make simple_process.upload
− Login to the device
− make login
26
Nodes and Applications in
Wireless Sensor Networks
− Sensor Networks consist of nodes with different
capabilities.
− Large number of heterogeneous sensor nodes
− Spread over a physical location
− It includes physical sensing, data processing and
networking
− In ad-hoc networks, sensors can join and leave
due to mobility, failure etc.
− Data can be processed in-network, or it can be
directly communicated to the endpoints.
Types of nodes
− Sensor nodes
− Low power
− Consist of sensing device, memory, processor and radio
− Resource-constrained
− Sink nodes
− Another sensor node or a different wireless node
− Normally more powerful/better resources
− Gateway
− A more powerful node
− Connection to core network
− Could consist service representation, cache/storage,
discovery and other functions
Types of applications
− Event detection
−
−
−
−
Reporting occurrences of events
Reporting abnormalities and changes
Could require collaboration of other nearby or remote nodes
Event definition and classification is an issue
− Periodic measurements
− Sensors periodically measure and report the observation and
measurement data
− Reporting period is application dependent
− Approximation and pattern detection
− Sending messages along the boundaries of patterns in both
space/time
− Tracking
− When the source of an event is mobile
− Sending event updates with location information
Requirements and challenges
− Fault tolerance
− The nodes can get damaged, run out of power, the
wireless communication between two nodes can be
interrupted, etc.
− To tolerate node failures, redundant deployments can be
necessary.
− Lifetime
− The nodes could have a limited energy supply;
− Sometimes replacing the energy sources is not practical
(e.g. underwater deployment, large/remote field
deployments).
− Energy efficient operation can be a necessity.
Requirements and challenges – Cont’d
− Scalability
− A WSN can consists of a large number of nodes
− The employed architectures and protocols should scale
to these numbers.
− Wide range of densities
− Density of the network can vary
− Different applications can have different node densities
− Density does not need to be homogeneous in the entire
network and network should adapt to such variations.
Requirements and challenges – Cont’d
− Programmability
− Nodes should be flexible and their tasks could change
− The programmes should be also changeable during
operation.
− Maintainability
− WSN and environment of a WSN can change;
− The system should be adaptable to the changes.
− The operational parameters can change to choose
different trade-offs (e.g. to provide lower quality when
energy efficiency is more important)
Required mechanisms
− Multi-hop wireless communications
− Communication over long distances can require
intermediary nodes as relay (instead of using high
transmission power for long range communications).
− Energy-efficient operation
− To support long lifetime
− Energy efficient communication/dissemination of
information
− Energy efficient determination of a requested
information
− Auto-configuration
− Self-xxx functionalities
− Tolerating node failures
− Integrating new nodes
Required mechanisms
− Collaboration and in-network processing
− In some applications a single sensor node is not able to handle
the given task or provide the requested information.
− Instead of sending the information form various source to an
external network/node, the information can be processed in
the network itself.
− e.g. data aggregation, summarisation and then propagating the
processed data with reduced size (hence improving energy
efficiency by reducing the amount of data to be transmitted).
− Data-centric
− Conventional networks often focus on sending data between
two specific nodes each equipped with an address.
− Here what is important is data and the observations and
measurements not the node that provides it.
Communication and
Network Protocol Support
Communication Protocols
− Wired
− USB, Ethernet
− Wireless
− Wifi, Bluetooth, ZigBee, IEEE 802.15.x
− Single-hop or multi-hop
− Sink nodes, cluster heads…
− Point-to-Point or Point-to-Multi Point
− (Energy) efficient routing
Wireless Communications
− Mostly performed in unlicensed bands according to open
standards
− Standard: IEEE 802.15.4 -Low Rate WPAN
− 868/915 MHz bands with transfer rates of 20 and 40 kbit/s, 2450
MHz band with a rate of 250 kbit/s
− Technology example: ZigBee
− Standard: ISO/IEC 18000-7 (standard for active RFID)
− 433 MHz unlicensed spectrum with transfer rates of 200kbit/s
− Technology example: Dash7
Adapted from: The Web of Things, Marko Grobelnik, Carolina Fortuna, Jožef Stefan Institute.
Wireless Communications - continued
− Standard: IEEE 802.15.1 –High Rate WPAN
− 2.40GHzbands with transfer rates of 1-24 Mbit/s
− Technology: Bluetooth (BT 3.0 Low Energy Mode)
− Standard: IEEE 802.11x –WLAN
− 2.4, 3.6 and 5GHz with transfer rates 15-150 Mbit/s
− Technology: WIFI
− Licensed bands
− Standard: 3GPP –WMAN, WWAN cellular communication
− 950 MHz, 1.8 and 2.1 GHz bands with data rate ranging from
20 Kbit/s to 7.2 Mbit/s, depending on the release
− Technology: GPRS, HSPA
Adapted from: The Web of Things, Marko Grobelnik, Carolina Fortuna, Jožef Stefan Institute.
IEEE 802.15.4 WPAN
− IEEE standard for WPAN applications
− It does not define other higher-level layers and
interoperability sub-layers are
− ZigBee is built on this standard
− TinyOS stack also uses some items of IEEE 802.15.4
hardware.
ZigBee
− It is supposed to be a low cost, low power mesh network protocol.
− ZigBee operation range is in the industrial, scientific and medical radio
bands;
− 868 MHz in Europe, 915 MHz in the USA and Australia and 2.4 GHz.
− ZigBee data transmission rates vary from:
− 20 kilobits/second in the 868 MHz frequency band to
− 250 kilobits/second in the 2.4 GHz frequency band.
− ZigBee’s physical layer and media access control defined in defined based
on the IEEE 802.15.4 standard.
− ZigBee nodes can go from sleep to active mode in 30 ms or less, the
latency can be low and in result the devices can be responsive, in
particular compared to Bluetooth devices that wake-up time can be longer
(typically around three seconds).
[source: Gary Legg, ZigBee: Wireless Technology for Low-Power Sensor Networks,
http://www.eetimes.com/document.asp?doc_id=1275760]
ZigBee
[source: Gary Legg, ZigBee: Wireless Technology for Low-Power Sensor Networks,
http://www.eetimes.com/document.asp?doc_id=1275760]
Network protocols
− The network (or OSI Layer 3 abstraction)
provides an abstraction of the physical world.
− Communication protocols
− Most of the IP-based communications are based on the
IPV.4 (and often via gateway middleware solutions)
− IP overhead makes it inefficient for embedded devices
with low bit rate and constrained power.
− However, IPv6.0 is increasingly being introduced for
embedded devices
− 6LowPAN
IPv6 over Low power Wireless Personal Area
Networks (6LowPAN)
− LoWPAN typically includes devices that work together to
connect the physical environment to real-world
applications, e.g., wireless sensors. LoWPANs conform to
the IEEE 802.15.4-2003 standard [IEEE802.15.4].
− Small packet size
− the maximum physical layer packet is 127 bytes
− 81 octets (81 * 8 bits) for data packets.
− Support for both 16-bit short or IEEE 64-bit extended
media access control addresses.
− Low bandwidth
− Data rates of 250 kbps, 40 kbps, and 20 kbps for each of the
currently defined physical layers (2.4 GHz, 915 MHz, and 868
MHz, respectively).
6LowPAN
− IPv6 requires the link to carry a payload of up to
1280 Bytes.
− Low-power radio links often do not support such
a large payload - IEEE 802.15.4 frame only
supports 127 Bytes of payload and around 80 B
in the worst case (with extended addressing and
full security information).
− the IPv6 base header, as shown, is relatively
large at 40 Bytes.
Source: Jonathan W. Hui and David E. Culler, IPv6 in Low-Power Wireless Networks, Proceedings of the IEEE (Volume:98 , Issue: 11 ).
6LowPAN
− To handle these issues, IPv6 over low-power
wireless personal area networks (6LoWPAN)
introduces an adaptation layer that sits at layer
2.5 (between the link and network layers).
− 6LoWPAN defines a header encoding to
support fragmentation when IPv6 datagrams do
not fit within a single frame and compresses IPv6
headers to reduce header overhead.
Source: Jonathan W. Hui and David E. Culler, IPv6 in Low-Power Wireless Networks, Proceedings of the IEEE (Volume:98 , Issue: 11 ).
Using gateway and middleware
− It is unlikely that everything will be IP enabled and/or will
run IP protocol stack
− Gateway and middleware solutions can interfaces between
low-level sensor island protocols and IP-based networks.
− The gateway can also provide other components such as
QoS support, caching, mechanisms to address
heterogeneity and interoperability issues.
Gateway and IP networks
Gateway
Frieder Ganz, Payam Barnaghi, Francois Carrez and Klaus Moessner, "Context-aware
Management for Sensor Networks", in the Fifth International Conference on COMmunication
System softWAre and middlewaRE (COMSWARE11), July 2011.
Services and Interfaces
Service interfaces to WSN
− Supporting high-level request/response interactions
− Asynchronous event notifications
− Identifying and accessing data
− By location, by observed entity,
− By semantically meaningful representations – “Room 35BA01”
− Accessibility of in-network processing functions
− Defining complex events
− Allow to specify Quality of Information requirements (e.g.
accuracy & timeliness requirements)
− Accessing node/network status information (e.g., battery
level)
− Security, management functionality, …
− There are emerging solutions and standards in this area
supported by Semantic Web technologies and Linked-data.
Service interfaces and Web connectivity
− WSN nodes are typically resource constrained
− Memory and process limitations
− Communication load
− Often none-IP or use 6LowPAN
− Using gateway and middleware is a clear solution
− Or can the nodes directly connect to the Web and
or support service interfaces?
Constrained Application Protocol (CoAP)
− CoAP is a transfer protocol for constrained nodes and
networks.
− CoAP uses the Representational State Transfer (REST)
architecture.
− REST make information available as resources that are
identified by URIs.
− Applications communication by exchanging representation of
these resources using a transfer protocol such as HTTP.
− Clients access servicer controlled resources using synchronous
request/response mechanisms.
− Such as GET, PUT, POST and DELETE.
− CoAp uses UDP instead of TCP and has a simple “message
layer” for re-transmitting lost packets.
− It also uses compression techniques.
C. Bormann, A. P. Castellani, Z. Shelby, "CoAP: An Application Protocol for Billions of Tiny Internet Nodes," IEEE Internet Computing,
vol. 16, no. 2, pp. 62-67, Feb. 2012, doi:10.1109/MIC.2012.29
Constrained Application Protocol (CoAP)
Server
200 OK
Txt/plain
17, Celsius
GET/temperature,
Room A
Client
CoAP protocol stack and interactions
C. Bormann, A. P. Castellani, Z. Shelby, "CoAP: An Application Protocol for Billions of Tiny Internet Nodes," IEEE Internet Computing,
vol. 16, no. 2, pp. 62-67, Feb. 2012, doi:10.1109/MIC.2012.29
Acknowledgment
− Parts of the content is adapted from:
− Waltenegus Dargie, Christian Poellabauer,
“Fundamentals of Wireless Sensor Networks: Theory and
Practice” (Wireless Communications and Mobile
Computing), Wiley, 2010.
− Holger Karl, Andreas Willig, “Protocols and Architectures
for Wireless Sensor Networks”, Wiley, 2007.
ISBN: 978-0-470-51923-3.
54
Questions?
55